1. Field of the Invention
This invention pertains generally to fluid pumps and mixers, more specifically to a miniaturized acoustic-fluidic pump or mixer.
2. Description of the Related Art
The oldest methods to generate flow in fluidic systems use external pumps of various types that are bulky and cannot be miniaturized. More recently, piezoelectrical driven membrane pumps less than 1 cm.times.1 cm.times.2 mm in size have been integrated into planar microfluidic systems. But these pumps require valves that can clog or otherwise fail. Miniature valve-less membrane pumps using fluidic rectifiers, such as the nozzle/diffuser and Telsa valve are under development, but rectifiers do not perform well in the laminar flow regime of microfluidics. They also have a pulsed flow that could be undesirable.
Elecroosmosis is a valve-less, no-moving parts pumping mechanism suitable for miniaturization and has been used for a number of microfluidic systems, often because of compatibility with electrophoretic separation. Electroosmosis depends on the proper wall materials, solution pH, and ionicity to develop a charged surface and an associated diffuse charged layer in the fluid about 10 nm thick. Application of an electric field along the capillary then drags the charged fluid layer next to the wall and the rest of the fluid with it so the velocity profile across the channel is flat, what is termed a "plug" profile. The greater drawbacks of electroosmosis are the wall material restrictions and the sensitivity of flow to fluid pH and ionicity. In addition, some large organic molecules and particulate matter such as cells can stick to the charged walls. Crosstalk can also be an issue for multichannel systems since the different channels are all electrically connected through the fluid. Finally, the velocity shear occurs in or near the diffuse charged layer and such strong shear could alter the form of large biological molecules near the wall.
The oldest methods of creating circulation or stirring in reservoirs move the fluid by the motion of objects such as vanes that in turn are driven by mechanical or magnetic means. The drawbacks for entirely mechanical systems are complications of coupling through reservoir walls with associated sealing or friction difficulties. The drawback to magnetic systems is in providing the appropriate magnetic fields without complicated external arrangements.
More recently, acoustic streaming has been used for promoting circulation in fluids. In Miyake et al., U.S. Pat. No.5,736,100, issued Apr. 7, 1998, provides a chemical analyzer non-contact stirrer using a single acoustic transducer unfocussed or focused using a geometry with a single steady acoustic beam directed to the center or the side of the reaction vessel to generate steady stirring. That patent, however, does not specify whether the flow is laminar or turbulent. Flow is laminar for microfluidics where the Reynolds numbers are less than 2000 and the very lack of turbulence makes mixing difficult. Nor does Miyake et al. address the production of non-steady mixing flows by multiple acoustic beams nor the higher frequencies necessary for maximum circulation for microfluidic reservoirs less than 1 cm in size. In laminar flow, two fluids of different composition can pass side-by-side and will not intermix except by diffusion. This mixing can be enhanced by non-steady multi-directional flows such as observed with bubble pumps.
Miniaturization offers numerous advantages in systems for chemical analysis and synthesis, such advantages include increased reaction and cooling rates, reduced power consumption and quantities of regents, and portability. Drawbacks include greater resistance to flow, clogging at constrictions and valves, and difficulties of mixing in the laminar flow regime.